RAMAN ANALYSIS APPARATUS AND METHODS

Abstract: Thermal conductivity of a sample (30), e.g. a diamond film, is measured by setting up a thermal gradient (38). This may be done, for example, using a heater (34) and heat sink (36). The temperature of a plurality of points (19) along the thermal gradient is then measured by illuminating them with a laser beam (10), and taking the ratio between the intensities of Stokes and anti-Stokes Raman scattering. Having determined the temperature of such points, the thermal gradient, is calculated, and from that the thermal conductivity.

Description: RAMAN ANALYSIS APPARATUS AND METHODS Field of the Invention This invention relates to spectroscopy. More particularly, it relates to the use of Raman spectroscopy in order to measure the thermal conductivity of a sample. Background of the Invention It is already known that the temperature of a sample can be measured using Raman spectroscopy. See for example U.S. Patent No. 4,081,215 (Penney), in which Raman spectroscopy is used to determine the temperature of a flame. It is also known to use Raman spectroscopy to determine the temperature at a point of a solid sample. However, it has not previously been known to use such techniques to determine the thermal conductivity of a sample. One material in which it is particularly desirable to measure thermal conductivity is diamond. It has recently become possible to grow diamond films with very high quality by chemical vapour deposition (CVD) . Such a film can be grown on a substrate, e.g. silicon, and used as a heat sink for high-powered semiconductor devices such as semiconductor lasers. For such a use, the ability to measure the thermal conductivity is obviously desirable. Summary of the Invention The present invention provides a method of measuring the thermal conductivity of a sample, comprising the steps of: locally heating the sample, thereby establishing a thermal gradient therein, measuring the temperature of at least a first part of the sample, and deriving therefrom the thermal conductivity of the sample, characterised in that in said temperature measuring step, the temperature is measured by causing Raman scattering of light into a spectrum having at least two Raman lines, detecting said two Raman lines, and determining said temperature from the ratio of the intensities of said lines. Brief Description of the Drawings Examples of apparatus and methods according to the invention will now be described with reference to the accompanying drawings, wherein: Fig 1 is a schematic diagram of Raman analysis apparatus, Fig 2 is a schematic cross-section through a sample, the thermal conductivity of which is to be measured, and Fig 3 is a diagrammatic representation of an image on a charge coupled device. Description of Preferred Embodiments Measurements of thermal conductivity may be made using apparatus which is similar to that described in European Patent Application No. 92310387.3 (publication no. EP 543578) and the corresponding U.S. application no. 07/976,513 and Japanese application no. 305,645/1992. Reference should be made to those earlier applications for full details of the apparatus, and they are incorporated herein by reference. However, the apparatus will be described briefly with reference to Fig 1. In Fig 1, an incoming laser beam 10 is reflected by a dichroic filter 12, and focused at 19 on a sample 18, by means of a microscope objective lens 16. Raman light scattered from the sample is collected by the lens 16, and passes through the dichroic filter 12 to a Raman analyser 20. As described in the above earlier applications, this may be a dispersive grating, or it may be a non-dispersive device which is tunable in use. In particular, the tunable non-dispersive device may be an interference filter, or an interference filter in series with a Fabry-Perot interferometer. After the Raman analyser 20, the Raman scattered light is focused by a lens 22 on to a photodetector, in the form of a charge coupled device (CCD) 24. This comprises a two- dimensional array of pixels. Data is acquired from the CCD 24 by a computer 25, which may also control the Raman analyser 20. Other photodetectors may be used instead of a CCD, and may or may not include a two-dimensional array of pixels. If the analyser 20 uses a dispersive diffraction grating, then the resulting Raman spectrum is dispersed across the CCD 24, as indicated at 28. Where a non-dispersive Raman analyser is used, then an image of the point 19 on the sample 18 is formed at 26 on the CCD, in Raman light at the wavenumber selected by the analyser. This apparatus can be used to measure thermal conductivity in several ways. The description below will refer to measuring the thermal conductivity of a diamond film, but it will be understood that the methods are equally applicable to other samples. Fig 2 shows a sample in which a diamond film 30 has been grown on a silicon substrate 32. At one end of the sample, there is provided a heater 34, while at the other end there is a heat sink 36. This is used to set up a known heat flow per unit area passing through the diamond film 30 in the direction of arrow 38. The heat flow per unit area may be designated Q/A, where Q is the heat flow and A is the cross-sectional area of the diamond film 30. In use, the sample is allowed to reach a steady state in which there is a temperature gradient dT/dx applied across the sample. Measuring this temperature gradient allows the thermal conductivity K to be calculated, using the formula K = (Q/A)/(dT/dx) (1) For the most accurate results, corrections may need to be made to allow for radiant heat loss from the surface of the sample and conduction through the substrate. The temperature gradient is measured by measuring the temperature at various points on the sample. This is done using the Raman apparatus of Fig 1. Use is made of the fact that the ratio R of the intensity I s of the Stokes Raman band to the intensity I a of the anti-Stokes band is very sensitive to temperature. For a vibrational mode of frequency v , R = I a /I s = exp(-hv/kT) (2) A simple method is to focus the incoming laser light 10 to a point 19 on the surface of the film 30, and then to track this point focus along the heat flow direction 38. This may be done by mounting the sample on a translational stage, controlled by the computer 25. A diffraction grating is preferred for the Raman analyser 20,since this disperses the Stokes and anti-Stokes peaks, at wavenumbers of ± 1332 cm "1 , across the CCD 24. The computer 25 can therefore obtain simultaneous readings of the intensities of both. A series of such readings of the two intensities is taken at subsequent points, as the point focus 19 tracks along the film 30. For each of these points, the computer stores the two intensities I a ,I s . Alternatively, if desired, the computer can calculate the value of the local temperature T from formula (2) in real time, as the scan proceeds, and just store this value. In either case, the computer subsequently calculates the temperature gradient dT/dx, knowing the distance by which the stage has been moved between each measurement point. It is then another easy step for the computer to use formula (1) to calculate the thermal conductivity K. This thermal conductivity may, of course, vary from one location in the sample to another, depending on the quality of the sample, and thus the measurement yields useful information about the sample quality. In another method, the microscope objective lens 16 is replaced by a cylindrical lens, to provide a line focus on the film 30. This line is normal to the plane of Fig 1, and lies along the direction 38 of the temperature gradient in Fig 2. Using a diffraction grating as the Raman analyser 20, Fig 3 shows diagrammatically the resulting image produced on the CCD 24. In Fig 3, reference numeral 40 indicates an image of the line which is focused on the film 30 (though if the dichroic filter 12 efficiently rejects the Rayleigh scattered light from the sample, no such line 40 may actually appear on the CCD 24) . For each point 42 on the line 40, a Raman spectrum is dispersed at right angles to the line 40, as indicated by the line 44. This will include the Stokes peak S and the anti-Stokes peak A for the diamond at the point in the film 30 corresponding to the point 42. For all other points in the line 40, there will be corresponding Stokes and anti-Stokes peaks S,A. It will be appreciated that the image shown in Fig 3 gives a snapshot of the temperature along the line focused on the sample at a given instant of time. The computer 25 reads in the data for the entire snapshot, determines for each point on the line 40 the intensities of the Stokes and anti-Stokes peaks S,A, and determines the temperature at that point 42 using formula (2) . As above, it works out the temperature gradient dT/dx, and determines the thermal conductivity K in various regions along the line 40 using formula (1) . It will be appreciated that both the above methods are quick and easy, particularly the second. Information about an area of the sample, instead of just a line in the direction 38, can be obtained in either of the above methods. This is achieved by mounting the sample on a translational stage which can scan in a direction perpendicular to the plane of Fig 2 (in addition to any scanning in the direction 38) . The method described above is repeated for each position of the stage. In any of the above methods, the diffraction grating in the Raman analyser 20 may be replaced by the interference filter and Fabry-Perot interferometer, placed in series as described in the earlier applications mentioned above. However, obtaining the Stokes and anti-Stokes peaks is now a two-stage procedure, since the filter and Fabry-Perot interferometer must be re-tuned from one peak to the other. Furthermore, the spectral resolution may not be as good as with the diffraction grating. In another method using the interference filter and Fabry- Perot interferometer, the incoming laser beam 10 is defocused (made to converge or diverge) so as to illuminate an area on the sample, instead of a point or line. A corresponding image of the illuminated area is now formed on the CCD 24, in light of the selected Raman wavenumber. The computer 25 reads in data for such an image in Raman light scattered in the Stokes band, and separately for Raman light scattered in the anti-Stokes band. For each point within the image, the temperature is determined by the computer, using formula (2) . The computer then calculates thermal conductivities K for all regions of interest within the illuminated area of the sample. The method just described clearly has the advantage of providing thermal conductivity over a two-dimensional area of the sample, without the need for scanning a translational stage. However, for reasons mentioned above we prefer to use a diffraction grating, with either a point or a line focus on the sample. A further method will now be described, which is less accurate than the methods above, but which does not require the heater 34. Instead, laser heating is used at a point to raise the local temperature of that point of the sample above its surroundings. The laser heating can be supplied by the same laser beam 10 which excites the Raman spectra, or by a separate, more powerful laser beam. The temperature reached at the point 19 which is being heated will be determined by the rate at which energy is absorbed, and by the thermal conductivity of the sample. Assuming that the rate of energy absorption is similar for different samples, the relative thermal conductivity of the samples can be determined by measuring and comparing the temperature at 19. The temperature is measured in the same way as discussed above, from the ratio of the intensities of the Stokes and anti-Stokes bands for diamond. Again, corrections should be made for radiant heat loss, for the most accurate results. Instead of heating a point 19, it is also possible to use a cylindrical lens for the lens 16, to provide local heating along a line focus. The temperature is measured at each point in the line as discussed above in relation to Fig 3. In another method, use is made of confocal techniques. Suitable confocal techniques are described in our International Application No. W092/22793, which is incorporated herein by reference. Alternatively, confocal action can be provided by a pinhole-type spatial filter, which is described as prior art in that International Application. It is possible to focus the laser beam 10 to a point within the film 30, e.g. as shown at 19A in Fig 2, instead of focusing it to a spot on the surface of the film 30. The confocal techniques then permit Raman spectra to be obtained which relate to the plane of the point 19A, excluding light which is scattered from other planes within the sample. The temperature at the point 19A is then determined as described above. By scanning in the direction of the arrow 38, thermal conductivity can be determined for regions within the sample, as well as at the surface of the sample. It is also possible to scan in the vertical direction of Fig 2, by adjusting the focal plane. These techniques enable one to determine how the temperature varies with depth in the film 30, thus yielding further useful information about the thermal conductivity. In any of the above methods, it is likely that the obtained Raman spectra will include not only Stokes and anti-Stokes peaks at ± 1332 cm '1 , characteristic of diamond, but also other peaks due to the silicon substrate 32. Thus, the temperature of the substrate 32 can also be determined, for example by looking at the Stokes and anti-Stokes peaks at ± 520 cm "1 , which are characteristic of silicon. This may be done at the same time as the determinations of the temperatures of the diamond film 30, and can yield useful information about the way in which the diamond film 30 conducts heat to or from the substrate 32. It would also be possible to measure the thermal conductivity of the substrate 32, if desired.

Claims: CLAIMS 1. A method of measuring the thermal conductivity of a sample, comprising the steps of:- locally heating the sample, thereby establishing a thermal gradient therein, measuring the temperature of at least a first part of the sample, and deriving therefrom the thermal conductivity of the sample, characterised in that in said temperature measuring step, the temperature is measured by causing Raman scattering of light into a spectrum having at least two Raman lines, detecting said two Raman lines, and determining said temperature from the ratio of the intensities of said lines. 2. A method according to claim 1, wherein the two Raman lines are a Stokes and an anti-Stokes Raman line. 3. A method according to claim 1 or claim 2, wherein said temperature measuring step takes place at at least two parts of the sample, which have said thermal gradient between them. 4. A method according to claim 3, including the step of scanning an illuminating light beam along the thermal gradient, to cause said Raman scattering at a multiplicity of said parts of the sample; and detecting said two Raman lines and determining said temperature from said ratio of the intensities at each of said multiplicity of parts of the sample. 5. A method according to claim 3, including the step of illuminating a line on the sample, along the thermal gradient, to cause said Raman scattering at a multiplicity of said parts of the sample along the line; and detecting said two Raman lines and determining said temperature from said ratio of the intensities at each of said multiplicity of parts of the sample. 6. A method according to any one of the preceding claims, wherein said Raman lines are detected by a photodetector comprising a two-dimensional array of pixels. 7. A method according to any one of the preceding claims, wherein said thermal gradient is produced by a heater at one part of the sample and a heat sink at another part of the sample. 8. A method according to any one of claims 1 to 6, wherein said local heating is produced by a laser beam. 9. A method according to anyone of the preceding claims, in which the sample comprises at least two different substances, and thermal conductivities are measured for each of said substances using Raman lines characteristic of the respective substances. 10. A method according to any one of the preceding claims, in which said Raman lines are detected confocally, to obtain Raman lines from one plane of the sample while excluding light scattered from other planes.

Inventors: BATCHELDER DAVID NEVILLE, HAYWARD IAN PAUL, PITT GILLIES DAVID, BATCHELDER, DAVID, NEVILLE, HAYWARD, IAN, PAUL, PITT, GILLIES, DAVID
Applicants: BATCHELDER DAVID NEVILLE, HAYWARD IAN PAUL, PITT GILLIES DAVID, RENISHAW PLC, BATCHELDER, DAVID, NEVILLE, HAYWARD, IAN, PAUL, PITT, GILLIES, DAVID
Application year: 1994
Main classification: G01N25
IPC codes: G01N21, G01N25, G01N33